1. Field
This patent specification relates generally to oilfield logging. More particularly, this patent specification relates to methods and systems for detecting gamma-rays in downhole applications.
2. Background
Many properties of a subterranean formation may be determined using different oilfield logging techniques, which may involve one or more tools having a radioisotope source. For example, to locate gas in a subterranean formation, a conventional practice combines data obtained from two tools. One of the tools is a “density” tool, which measures the electron density of the formation, and the other of the tools is a “neutron porosity” tool, which generally measures the density of hydrogen in the formation, known as the “hydrogen index” (HI). Based on measurements of formation density and hydrogen index, the porosity and pore fluid density of the formation may be determined. For a given formation fluid density, or gas saturation, a combination of a decrease in the formation density and an increase in the hydrogen index indicates an increase in the porosity of the formation. Meanwhile, for a given formation porosity, a combination of a decrease in the formation density and a decrease in hydrogen index indicates a decrease in the pore fluid density and hydrogen content. For pores filled with water and gas or oil and gas, the density and hydrogen index are an indication of the gas saturation (volume fraction of the pores occupied by gas). For pores filled with gas only, the density and hydrogen index are an indication of gas density (pressure).
The density and neutron porosity tools for measuring formation density and hydrogen index may generally employ radioactive sources to obtain formation density and hydrogen index measurements, respectively. For example, the density tool may use a source such as 137Cs to emit gamma-rays into a formation. Based on a count of gamma-rays scattered by the formation, the density tool may determine the electron density of the formation. Similarly, the neutron porosity tool may use a source such as 241Am—Be to emit neutrons into a formation. A count of neutrons scattered by the formation may yield a hydrogen index measurement. Such radioisotope sources may be disadvantageous in oilfield tools, as the sources may be heavily regulated by law and their output may diminish over time.
In lieu of such radioisotope sources, an electronic neutron generator may produce both neutrons and gamma-rays. To do so, the electronic neutron generator may emit neutrons into a formation, which may in turn produce gamma-rays via inelastic scattering and neutron capture events. A count of gamma-rays produced by inelastic scattering may generally yield a signal that corresponds to formation density, and a count of scattered neutrons may generally yield a neutron porosity signal that corresponds to the hydrogen index of the formation. However, a count of gamma-rays produced by neutron capture may also yield a signal corresponding to the hydrogen index of the formation. Thus, as a count of gamma-rays produced by neutron capture events may overwhelm a count of gamma-rays produced by inelastic scattering, simply counting all scattered gamma-rays may yield a signal that corresponds, at least in part, to the hydrogen index of the formation. Since such gamma-ray and neutron counts are not independent, the two signals may not enable determination of porosity and gas saturation. Other oilfield logging techniques may involve spectral analyses of both the inelastic and capture gamma-rays produced by neutrons emitted into a formation. As noted above, it may be possible for detected inelastic gamma-rays to be overwhelmed by detected neutron capture gamma-rays.
The selection of an optimum detector type for, capture spectroscopy, inelastic measurements, and density measurement has been an issue requiring many trade-offs. The trade-offs that must be considered include not only the hardware, but how the collected data is processed.
A property that is usually ignored for the inelastic measurement is the affinity the detector has for interacting with neutrons of various energies, including thermal, epithermal, and fast and the production and detection of associated particles resulting from these interactions. For some previous and existing inelastic measurement tools, the number of detected events from these neutron reactions can be over 60% of the total detected counts. This results in a huge unwanted background signal that significantly degrades the performance of the desired signal, since the unwanted signal must be removed in some way. The degradation in performance can be due to reduced precision (logging speed) or complicating the interpretation of the physics of the measurements such that petrophysical usage is limited.
Paper “Response of the Carbon/Oxygen Measurement for an Inelastic Gamma Ray Spectroscopy Tool, B. A. Roscoe and J. A. Grau, SPE 14460, SPE Formation Evaluation, March (1988) 76-80 and A New Through-Tubing Oil-Saturation Measurement System”, B. A. Roscoe, C. Stoller, R. A. Adolph, Y. Boutemy, J. C. Cheeseborough, III, J. S. Hall, D. C. McKeon, D. Pittman, B. Seeman, and S. R. Thomas, SPE 21413, presented to the SPE International Arctic Technology Conference, Anchorage, Ak., May 29-31, 1991; presented to the Middle East Oil Show & Conference, Bahrain, Nov. 16-19, 1991 showed the related effects previously discussed on the measurements (biases) and demonstrated that the bias effects on the measurement were addressed.
The root cause of the problem has to do with the component materials used in the radiation detectors where these materials have a high affinity for interacting with neutrons of various energies, including thermal, epithermal, and fast and the production and detection of associated particles resulting from these interactions. In the early 80's, the best downhole detector meeting the needs of the inelastic measurement was NaI, which has a very high affinity for neutrons of all energies (fast, thermal, and epithermal). In the late 80's gadolinium oxyorthosilicate (GSO or Gd2SiO5) became available, which did not have the problematic fast component other than silicon and oxygen, which were already present in the formation. However, it had a very large thermal and epithermal neutron component. Bismuth germanate (BGO or Bi4Ge3012) also became available, which had minimal neutron response, but did not operate at elevated temperatures.
Classically, the thermal neutron component has been removed by surrounding the detector with a thermal neutron absorbing material, such as boron, as disclosed in U.S. Pat. No. 4,937,446 for the RST tool of Schlumberger Technology Corporation. The RST tool design included sufficient boron to remove all of the thermal neutron signal, and as much as reasonably possible, the epithermal neutron signal.
Down-hole density measurements using a sourceless technology have been proposed. With sourceless technology, the radiation used to produce the measurement will be produced from an electronic source. The change of this source type yields many benefits, but it introduces other issues that must be addressed in order to get full benefit from this technology.
One of those issues relates to handling the very high instantaneous count rate that can be present. Actually, the instantaneous detector countrate in a tool utilizing an accelerator source can be much higher than conventional tools for different reasons. First, for a radiochemical source, higher countrates are achieved by using a higher activity source so transportation issues are more restrictive. For regulatory concerns still, the maximum activity of these source has to be limited. An accelerator source can be turned off, so the radiation safety issues of transportation are minimized. For an accelerator source, the activity can be turned up, or down, electronically. Second, some accelerator technologies may need to operate, or the measurement may want to operate in, a pulsed mode, where all the radiation comes out in a short period of time, e.g. 5 μs. This means that during this burst of radiation, the detector must operate at a very high instantaneous countrate which many of the standard detectors cannot handle.
Therefore, there is a need for detectors that can handle very high instantaneous countrates, and still give a signal with the required signal to noise.